The present invention relates to an improved process for preparing non-agglomerated water insoluble inorganic particles having a dense amorphous silica coating by using a dispersion aid.
U.S. Pat. No. 2,885,366 describes several methods for applying a dense silica coating to substrate particles, i.e., core particles by suspending the particles in water and then coating with active silica while maintaining the pH in the range of 7 to 11. If the substrate particles in the suspension are in some degree aggregated together to form clumps or agglomerates these are coated with a layer of dense amorphous silica as well as any primary substrate particles. Such a product embraces a wide distribution of sizes of silica coated substrates. Although the coated agglomerates can be broken down into smaller fragments, by milling or sonification, the fragmented agglomerates have exposed surfaces which are not coated with silica. This detracts from the performance of these materials in many applications, e.g., rough surface appearance in films such as paints.
U.S. Pat. Re. No. 27,818 describes this problem for silica coated titanium dioxide pigments. The solution offered in that reference was treating a suspension of silica coated titanium dioxide with an aqueous solution of a water soluble aluminum compound. Paints made using this pigment gave smoother films.
The use of dispersion aids for initially preparing non-agglomerated aqueous suspensions of particulate substrates is known in the art. There is no previous mention, however, of using suitable dispersion aids in the silica coating process to minimize particle agglomeration and give a more uniform particulate product which has been accomplished by the present invention. The products produced by the present improved process have shown particle size improvement and superior performance such as decreased surface roughness when used as pigments in film formers, e.g., paints.